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# Chapter 2 - Control Structures

Outline 2.1 Introduction 2.2 Algorithms 2.3 Pseudocode 2.4 Control Structures 2.5 if Selection Structure 2.6 if/else Selection Structure 2.7 while Repetition Structure 2.8 Formulating Algorithms: Case Study 1 (Counter-Controlled Repetition)

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## Chapter 2 - Control Structures

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1. Outline 2.1 Introduction 2.2 Algorithms 2.3 Pseudocode 2.4 Control Structures 2.5 if Selection Structure 2.6 if/else Selection Structure 2.7 while Repetition Structure 2.8 Formulating Algorithms: Case Study 1 (Counter-Controlled Repetition) 2.9 Formulating Algorithms with Top-Down, Stepwise Refinement: Case Study 2 (Sentinel-Controlled Repetition) 2.10 Formulating Algorithms with Top-Down, Stepwise Refinement: Case Study 3 (Nested Control Structures) 2.11 Assignment Operators 2.12 Increment and Decrement Operators 2.13 Essentials of Counter-Controlled Repetition 2.14 for Repetition Structure 2.15 Examples Using the for Structure Chapter 2 - Control Structures

2. Chapter 2 - Control Structures Outline 2.16 switch Multiple-Selection Structure 2.17 do/while Repetition Structure 2.18 break and continue Statements 2.19 Logical Operators 2.20 Confusing Equality (==) and Assignment (=) Operators 2.21 Structured-Programming Summary

3. 2.1Introduction • Before writing a program • Have a thorough understanding of problem • Carefully plan your approach for solving it • While writing a program • Know what “building blocks” are available • Use good programming principles

4. 2.2 Algorithms • Computing problems • Solved by executing a series of actions in a specific order • Algorithm a procedure determining • Actions to be executed • Order to be executed • Example: recipe • Program control • Specifies the order in which statements are executed

5. 2.3 Pseudocode • Pseudocode • Artificial, informal language used to develop algorithms • Similar to everyday English • Not executed on computers • Used to think out program before coding • Easy to convert into C++ program • Only executable statements • No need to declare variables

6. 2.4 Control Structures • Sequential execution • Statements executed in order • Transfer of control • Next statement executed not next one in sequence • 3 control structures (Bohm and Jacopini) • Sequence structure • Programs executed sequentially by default • Selection structures • if, if/else, switch • Repetition structures • while, do/while, for

7. 2.4 Control Structures • C++ keywords • Cannot be used as identifiers or variable names

8. 2.4 Control Structures • Flowchart • Graphical representation of an algorithm • Special-purpose symbols connected by arrows (flowlines) • Rectangle symbol (action symbol) • Any type of action • Oval symbol • Beginning or end of a program, or a section of code (circles) • Single-entry/single-exit control structures • Connect exit point of one to entry point of the next • Control structure stacking

9. 2.5 if Selection Structure • Selection structure • Choose among alternative courses of action • Pseudocode example: If student’s grade is greater than or equal to 60 Print “Passed” • If the condition is true • Print statement executed, program continues to next statement • If the condition is false • Print statement ignored, program continues • Indenting makes programs easier to read • C++ ignores whitespace characters (tabs, spaces, etc.)

10. 2.5 if Selection Structure • Translation into C++ If student’s grade is greater than or equal to 60 Print “Passed” if ( grade >= 60 ) cout << "Passed"; • Diamond symbol (decision symbol) • Indicates decision is to be made • Contains an expression that can be true or false • Test condition, follow path • if structure • Single-entry/single-exit

11. A decision can be made on any expression. zero - false nonzero - true Example: 3 - 4 istrue true false print “Passed” grade >= 60 2.5 if Selection Structure • Flowchart of pseudocode statement

12. 2.6 if/else Selection Structure • if • Performs action if condition true • if/else • Different actions if conditions true or false • Pseudocode if student’s grade is greater than or equal to 60print “Passed” else print “Failed” • C++ code if ( grade >= 60 ) cout << "Passed";else cout << "Failed";

13. Condition Value if true Value if false false true print “Passed” print “Failed” grade >= 60 2.6 if/else Selection Structure • Ternary conditional operator (?:) • Three arguments (condition, value if true, value if false) • Code could be written: cout << ( grade >= 60 ? “Passed” : “Failed” );

14. 2.6if/else Selection Structure • Nested if/else structures • One inside another, test for multiple cases • Once condition met, other statements skipped if student’s grade is greater than or equal to 90 Print “A” else if student’s grade is greater than or equal to 80 Print “B” else if student’s grade is greater than or equal to 70 Print “C” else if student’s grade is greater than or equal to 60 Print “D” else Print “F”

15. 2.6if/else Selection Structure • Example if ( grade >= 90 ) // 90 and above cout << "A";else if ( grade >= 80 ) // 80-89 cout << "B";else if ( grade >= 70 ) // 70-79 cout << "C"; else if ( grade >= 60 ) // 60-69 cout << "D";else // less than 60 cout << "F";

16. 2.6if/else Selection Structure • Compound statement • Set of statements within a pair of braces if ( grade >= 60 ) cout << "Passed.\n"; else { cout << "Failed.\n"; cout << "You must take this course again.\n";} • Without braces, cout << "You must take this course again.\n"; always executed • Block • Set of statements within braces

17. 2.7 while Repetition Structure • Repetition structure • Action repeated while some condition remains true • Psuedocode while there are more items on my shopping list Purchase next item and cross it off my list • while loop repeated until condition becomes false • Example int product = 2; while ( product <= 1000 ) product = 2 * product;

18. true product <= 1000 product = 2 * product false 2.7 The while Repetition Structure • Flowchart of while loop

19. 2.8 Formulating Algorithms (Counter-Controlled Repetition) • Counter-controlled repetition • Loop repeated until counter reaches certain value • Definite repetition • Number of repetitions known • Example A class of ten students took a quiz. The grades (integers in the range 0 to 100) for this quiz are available to you. Determine the class average on the quiz.

20. 2.8 Formulating Algorithms (Counter-Controlled Repetition) • Pseudocode for example: Set total to zero Set grade counter to one While grade counter is less than or equal to ten Input the next grade Add the grade into the total Add one to the grade counter Set the class average to the total divided by tenPrint the class average • Next: C++ code for this example

21. 1 // Fig. 2.7: fig02_07.cpp 2 // Class average program with counter-controlled repetition. 3 #include <iostream> 4 5 using std::cout; 6 using std::cin; 7 using std::endl; 8 9 // function main begins program execution 10 int main() 11 { 12 int total; // sum of grades input by user 13 int gradeCounter; // number of grade to be entered next 14 int grade; // grade value 15 int average; // average of grades 16 17 // initialization phase 18 total = 0; // initialize total 19 gradeCounter = 1; // initialize loop counter 20 fig02_07.cpp(1 of 2)

23. 2.9 Formulating Algorithms (Sentinel-Controlled Repetition) • Suppose problem becomes: Develop a class-averaging program that will process an arbitrary number of grades each time the program is run • Unknown number of students • How will program know when to end? • Sentinel value • Indicates “end of data entry” • Loop ends when sentinel input • Sentinel chosen so it cannot be confused with regular input • -1 in this case

24. 2.9 Formulating Algorithms (Sentinel-Controlled Repetition) • Top-down, stepwise refinement • Begin with pseudocode representation of top Determine the class average for the quiz • Divide top into smaller tasks, list in order Initialize variables Input, sum and count the quiz grades Calculate and print the class average

25. 2.9 Formulating Algorithms (Sentinel-Controlled Repetition) • Many programs have three phases • Initialization • Initializes the program variables • Processing • Input data, adjusts program variables • Termination • Calculate and print the final results • Helps break up programs for top-down refinement

26. 2.9 Formulating Algorithms (Sentinel-Controlled Repetition) • Refine the initialization phase Initialize variables goes to Initialize total to zero Initialize counter to zero • Processing Input, sum and count the quiz grades goes to Input the first grade (possibly the sentinel) While the user has not as yet entered the sentinel Add this grade into the running total Add one to the grade counter Input the next grade (possibly the sentinel)

27. 2.9 Formulating Algorithms (Sentinel-Controlled Repetition) • Termination Calculate and print the class average goes to If the counter is not equal to zero Set the average to the total divided by the counter Print the average Else Print “No grades were entered” • Next: C++ program

28. 1 // Fig. 2.9: fig02_09.cpp 2 // Class average program with sentinel-controlled repetition. 3 #include <iostream> 4 5 using std::cout; 6 using std::cin; 7 using std::endl; 8 using std::fixed; 9 10 #include <iomanip> // parameterized stream manipulators 11 12 using std::setprecision; // sets numeric output precision 13 14 // function main begins program execution 15 int main() 16 { 17 int total; // sum of grades 18 int gradeCounter; // number of grades entered 19 int grade; // grade value 20 21 double average; // number with decimal point for average 22 23 // initialization phase 24 total = 0; // initialize total 25 gradeCounter = 0; // initialize loop counter Data type double used to represent decimal numbers. fig02_09.cpp(1 of 3)

31. 2.10 Nested Control Structures • Problem statement A college has a list of test results (1 = pass, 2 = fail) for 10 students. Write a program that analyzes the results. If more than 8 students pass, print "Raise Tuition". • Notice that • Program processes 10 results • Fixed number, use counter-controlled loop • Two counters can be used • One counts number that passed • Another counts number that fail • Each test result is 1 or 2 • If not 1, assume 2

32. 2.10 Nested Control Structures • Top level outline Analyze exam results and decide if tuition should be raised • First refinement Initialize variables Input the ten quiz grades and count passes and failures Print a summary of the exam results and decide if tuition should be raised • Refine Initialize variables to Initialize passes to zero Initialize failures to zero Initialize student counter to one

33. 2.10 Nested Control Structures • Refine Input the ten quiz grades and count passes and failures to While student counter is less than or equal to tenInput the next exam result If the student passed Add one to passesElse Add one to failures Add one to student counter

34. 2.10 Nested Control Structures • Refine Print a summary of the exam results and decide if tuition should be raised to Print the number of passes Print the number of failures If more than eight students passed Print “Raise tuition” • Program next

35. 1 // Fig. 2.11: fig02_11.cpp 2 // Analysis of examination results. 3 #include <iostream> 4 5 using std::cout; 6 using std::cin; 7 using std::endl; 8 9 // function main begins program execution 10 int main() 11 { 12 // initialize variables in declarations 13 int passes = 0; // number of passes 14 int failures = 0; // number of failures 15 int studentCounter = 1; // student counter 16 int result; // one exam result 17 18 // process 10 students using counter-controlled loop 19 while ( studentCounter <= 10 ) { 20 21 // prompt user for input and obtain value from user 22 cout << "Enter result (1 = pass, 2 = fail): "; 23 cin >> result; 24 fig02_11.cpp(1 of 2)

36. 25 // if result 1, increment passes; if/else nested in while 26 if ( result == 1 ) // if/else nested in while 27 passes = passes + 1; 28 29 else// if result not 1, increment failures 30 failures = failures + 1; 31 32 // increment studentCounter so loop eventually terminates 33 studentCounter = studentCounter + 1; 34 35 } // end while 36 37 // termination phase; display number of passes and failures 38 cout << "Passed " << passes << endl; 39 cout << "Failed " << failures << endl; 40 41 // if more than eight students passed, print "raise tuition" 42 if ( passes > 8 ) 43 cout << "Raise tuition " << endl; 44 45 return0; // successful termination 46 47 } // end function main fig02_11.cpp(2 of 2)

37. Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 2 Enter result (1 = pass, 2 = fail): 2 Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 2 Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 2 Passed 6 Failed 4 Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 2 Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 1 Passed 9 Failed 1 Raise tuition fig02_11.cppoutput (1 of 1)

38. 2.11 Assignment Operators • Assignment expression abbreviations • Addition assignment operator c = c + 3; abbreviated to c += 3; • Statements of the form variable = variable operator expression; can be rewritten as variable operator= expression; • Other assignment operators d -= 4 (d = d - 4) e *= 5 (e = e * 5) f /= 3 (f = f / 3) g %= 9 (g = g % 9)

39. 2.12 Increment and Decrement Operators • Increment operator (++) - can be used instead of c += 1 • Decrement operator (--) - can be used instead of c -= 1 • Preincrement • When the operator is used before the variable (++c or –c) • Variable is changed, then the expression it is in is evaluated. • Posincrement • When the operator is used after the variable (c++ or c--) • Expression the variable is in executes, then the variable is changed.

40. 2.12 Increment and Decrement Operators • Increment operator (++) • Increment variable by one • c++ • Same as c += 1 • Decrement operator (--) similar • Decrement variable by one • c--

41. 2.12 Increment and Decrement Operators • Preincrement • Variable changed before used in expression • Operator before variable (++c or --c) • Postincrement • Incremented changed after expression • Operator after variable (c++, c--)

42. 2.12 Increment and Decrement Operators • If c = 5, then • cout << ++c; • c is changed to 6, then printed out • cout << c++; • Prints out 5 (cout is executed before the increment. • c then becomes 6

43. 2.12 Increment and Decrement Operators • When variable not in expression • Preincrementing and postincrementing have same effect ++c; cout << c; and c++; cout << c; are the same

44. 1 // Fig. 2.14: fig02_14.cpp 2 // Preincrementing and postincrementing. 3 #include <iostream> 4 5 using std::cout; 6 using std::endl; 7 8 // function main begins program execution 9 int main() 10 { 11 int c; // declare variable 12 13 // demonstrate postincrement 14 c = 5; // assign 5 to c 15 cout << c << endl; // print 5 16 cout << c++ << endl; // print 5 then postincrement 17 cout << c << endl << endl; // print 6 18 19 // demonstrate preincrement 20 c = 5; // assign 5 to c 21 cout << c << endl; // print 5 22 cout << ++c << endl; // preincrement then print 6 23 cout << c << endl; // print 6 fig02_14.cpp(1 of 2)

45. 24 25 return0; // indicate successful termination 26 27 } // end function main fig02_14.cpp(2 of 2)fig02_14.cppoutput (1 of 1) 5 5 6 5 6 6

46. 2.13 Essentials of Counter-Controlled Repetition • Counter-controlled repetition requires • Name of control variable/loop counter • Initial value of control variable • Condition to test for final value • Increment/decrement to modify control variable when looping

47. 1 // Fig. 2.16: fig02_16.cpp 2 // Counter-controlled repetition. 3 #include <iostream> 4 5 using std::cout; 6 using std::endl; 7 8 // function main begins program execution 9 int main() 10 { 11 int counter = 1; // initialization 12 13 while ( counter <= 10 ) { // repetition condition 14 cout << counter << endl; // display counter 15 ++counter; // increment 16 17 } // end while 18 19 return0; // indicate successful termination 20 21 } // end function main fig02_16.cpp(1 of 1)

48. 1 2 3 4 5 6 7 8 9 10 fig02_16.cppoutput (1 of 1)

49. 2.13 Essentials of Counter-Controlled Repetition • The declaration int counter = 1; • Names counter • Declares counter to be an integer • Reserves space for counter in memory • Sets counter to an initial value of 1

50. 2.14 for Repetition Structure • General format when using for loops for ( initialization; LoopContinuationTest; increment ) statement • Example for( int counter = 1; counter <= 10; counter++ ) cout << counter << endl; • Prints integers from one to ten No semicolon after last statement

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